1. Field of the Invention
The present invention relates to an optical router which is disposed on an optical node basis, selects optical paths for optical signals transmitting them through such optical transmission lines as optical fibers, and outputs the optical signals through the optical paths, whereby an optical communications network is formed. More specifically, the present invention relates to an optical router capable of efficiently allocating optimum optical paths to the optical signals at optimum timings.
2. Description of the Prior Art
An optical router is disposed on an optical node basis, selects the optical path of an optical signal transmitting it through such an optical transmission line as an optical fiber, and outputs the optical signal through the optical path, whereby an optical communications network is formed. Publications on the prior art related to optical routers include the following:                Japanese Laid-open Patent Application 1996-163610        Japanese Laid-open Patent Application 1996-204675        Japanese Laid-open Patent Application 2001-255567        Japanese Laid-open Patent Application 2001-255084        Japanese Laid-open Patent Application 2002-164847        “Photonic Network Revolution  Technologies for realizing the world's most advanced information technology nation”: published by the Secretariat, Photonic Internet Forum, within The Support Center for Advanced Telecommunications Technology Research, Foundation (January 2002): 95–98        
FIG. 1 is a block diagram illustrating an example of such a conventional optical router as mentioned above, where the example is identical to the optical router described in the patent application 2002-284970 filed by the applicant of the application concerned.
The patent application 2002-284970 filed by the applicant of the application concerned illustrates an example of an optical router wherein an optical signal (optical packet signal) being transmitted is split into a header part and a data part, and routing information, such as a destination address, is added to the header part according to the routing information, thereby permitting a selection to be made from given optical paths.
FIG. 1 indicates optical delay means 1 for delaying optical input signals by desired lengths of time by circulating (transmitting) the signals through a fiber-optic optical loop as many times as desired, an optical-electrical converter 2, such as a photodiode or phototransistor, optical switch 3 provided with three input ports and three output ports, controller 4 for controlling optical path selection made by optical switch 3; and memory 5 wherein path control information, such as routing tables, is stored. In addition, optical delay means 1, optical-electrical converter 2, optical switch 3, controller 4 and memory 5 compose optical router 50.
In FIG. 1, the three optical input signals indicated by SG01 are input to the three input ends of optical delay means 1, as well as to the three input ends of optical-electrical converter 2.
Optical output signals from the three output ends of optical delay means 1 are input to the three input ports of optical switch 3, and the three optical output signals indicated by SG02 in FIG. 1 are output from the three output ports of optical switch 5.
The electrical output signal of optical-electrical converter 2 is coupled with controller 4, and the electrical delay control signal of controller 4 and the electrical routing control signal thereof indicated by SS01 are coupled with the control terminals of optical delay means 1 and optical switch 3. In addition, the electrical input-output signal of controller 4 is mutually coupled with memory 5.
Now the behavior of the example of the prior art optical router illustrated in FIG. 1 is described by referring to FIG. 2. FIG. 2 is a schematic view illustrating a specific example of optical delay means 1 and indicates an optical switch 6 and optical fiber 7 composing the optical loop.
The optical input signals indicated by SG01 in FIG. 1, which contain routing information, such as destination addresses, added to the header parts thereof, are delayed at optical delay means 1 by desired lengths of time just long enough for controller 4 or any other element to process electrical signals.
For example, optical switch 6 is controlled so that the path of an optical input signal is changed, the signal is entered to the optical loop comprised of optical fiber 7, and the optical input signal is transmitted while being circulated as indicated by RP11 in FIG. 2. If optical switch 6 is controlled under this condition so that the optical input signal is confined within the optical loop, a delay time, which is as long as the duration required for the signal to circulate (transmit) through optical fiber 7, occurs.
Consequently, by circulating (transmitting) the optical input signal through the optical loop as many times as desired and controlling optical switch 6 to let the signal out of the optical loop so that the delay time is properly adjusted, it is possible to delay the optical input signal by lengths of time just long enough for controller 4 or any other element to process electrical signals.
Concurrently, the optical input signals indicated by SG01 in FIG. 1, which contain routing information, such as destination addresses, added to the header parts thereof, are converted to electrical signals at optical-electrical converter 2 and input to controller 4.
Controller 4 extracts the routing information from the electrical signal being input from optical-electrical converter 2, finds path control information stored in memory 5 according to the routing information, specifies a subsequent-stage optical router (output port) appropriate for the entered optical signals to transmit the signal to the destination through the shortest path, and accordingly selects from the optical paths of optical switch 3 by outputting the electrical routing control signal indicated by SS01 in FIG. 1.
For example, controller 4 controls optical switch 3 so that an optical path is selected in such a manner that an optical input signal is input to the input port of optical switch 5 indicated by PT01 in FIG. 1, and is output from the output port of optical switch 3 indicated by PT02.
If such an optical input signal as is properly delayed by optical delay means 1 after the completion of such optical path selection as described above is input to the input port of optical switch 3 indicated by PT01, the optical output signal will be output from the output port indicated by PT02.
This means that by adding routing information, such as a destination address, to the header part of an optical signal, it is possible to make optical path selections according to the routing information.
Furthermore, since it is possible for optical delay means 1 to delay the optical input signals by desired lengths of time just long enough for controller 4 or any other element to process electrical signals, it is also possible to cope with such problems as the occurrence of large delay times resulting from the failure to transfer the optical input signals (optical packets).
However, since the example of the prior art optical router is configured so that the optical input signals are circulated (transmitted) through the optical loop comprised of optical fiber 7 as many times as desired using optical delay means 1 and optical switch 6 is controlled to let the signals out of the optical loop so that the delay time is properly adjusted, the delay time has discrete values.
Let us take FIG. 3, which is a timing diagram illustrating the way delay times are produced by optical delay means 1, as an example. In FIG. 3, optical signal (no delay) (a) delays changing optical signal (one-turn delay) (b), optical signal (two-turn delay) (c), optical signal (three-turn delay) (d) and optical signal (four-turn delay) (e) step by step, as the number of circulations (turns) at the optical loop comprised of optical fiber 7 increases to one circulation (turn), two circulations (turns), three circulations (turns) and four circulations (turns).
More specifically, a delay time indicated by TD21 in FIG. 3 is produced in the first turn, a delay time indicated by TD22 is produced in the second turn, a delay time indicated by TD23 is produced in the third turn, and a delay time indicated by TD24 is produced in the fourth turn. This means that a total delay time of “TD21+TD22+TD23+TD24” is produced as the result of making four turns.
Note that the delay time produced by a single turn is kept constant if the ambient environmental conditions (temperature, etc.) of optical fiber 7 composing the optical loop remain unchanged, and the following equation holds true:TD21=TD22=TD23=TD24  (1)
For this reason, a delay time produced by optical delay means 1 equals an integer multiple of TD21 (number of circulations or turns), resulting in a discrete value.
On the other hand, the length of time just enough for controller 4 or any other element to process electrical signals greatly varies depending on whether or not forwarding error correction processing (hereinafter simply referred to as FEC processing) is applied. The length of time also varies greatly depending on the algorithm used even when FEC processing is applied.
Note however that as discussed earlier, the delay time resolution of optical delay means 1 is TD21 shown in FIG. 3. It is therefore not possible to fine-tune the delay time in increments smaller than TD21.
Let us take FIG. 4 as an example, which is a timing diagram illustrating the relationship between optical signals and an electrical routing control signal. FIG. 4 shows optical signal (no delay) (a), optical signal (one-turn delay) (b) whose number of circulations (turns) at the optical loop comprised of optical fiber 7 is one, and optical signal (two-turn delay) (c) whose number of circulations (turns) is two.
In FIG. 4, the time length indicated by PT31 in electrical routing control signal (d) is just long enough for controller 4 or any other element to process electrical signals. In order to secure such a time length, the optical signal in question must be delayed at optical delay means 1 by letting the signal undergo at least two circulations (turns), rather than one circulation (turn).
In this case, a dead time indicated by WT31 in FIG. 4 occurs during the time interval from when the required processing of electrical signals is completed at controller 4 or any other element within the time length indicated by PT31, to when controller 4 selects from the optical paths of optical switch 3 and the optical signal is delayed by TD21+TD22 before being allowed to enter optical switch 3.
This means that the optical paths of optical switch 3 are occupied during the time length indicated by WT31 in FIG. 4 even though none of the optical paths is used (no optical signals are transmitted). Consequently, the prior art optical router has had the problem that it is not possible to efficiently allocate optimum optical paths to optical signals (optical packets) at optimum timings.
Since the delay time indicated by TD21 in FIG. 4, for example, is extended in cases where the optical loop of optical delay means 1 is relatively long, the dead time indicated by WT31 is likely to increase further.